(a) Field of the Invention
The present invention relates to a heating resistor of a single crystal manufacturing apparatus which is used for manufacturing at high yield a high-quality single crystal which is free from any crystal defect.
(b) Description of the Prior Art
Conventionally, most industrial single crystals are manufactured by methods wherein a crystal material is heated and melted in a melting pot, and a seed crystal is brought into contact with the melt and is then pulled upward while being rotated.
Such methods include the Czochralski method (to be referred to as the CZ method hereinafter) and the liquid encapsulated Czochralski method (to be referred to as the LEC method hereinafter) for manufacturing single crystals containing an element having a high vapor pressure.
In this case, an RF coil or heating resistor is used as a heating medium. However, such a heating resistor is more frequently used to manufacture monocrystalline semiconductors, and is designed to heat the melting pot which is filled with the crystal material from the side surface thereof. For example, a heating resistor used for a monocrystalline silicon manufacturing apparatus for use with the CZ method is generally disposed on the side surface wall portion of the melting pot. On the other hand, a heating resistor used for a monocrystalline GaP or GaAs manufacturing apparatus for use with the LEC method has a side wall portion which coaxially surrounds the melting pot and a bottom portion which supports the side wall portion. It should be noted, however, that the bottom portion is designed only to support the side wall portion.
The latter heating resistor will be described with reference to FIG. 1.
A side wall portion 1 of the heating resistor has a tapered shape. A thickness dW of the side wall portion 1 is greatest at the upper edge thereof. However, a thickness dB of a bottom portion 2 is smallest at a connecting portion between the side portion 1 and the bottom portion 2. The thickness dB then increases in a direction toward a connecting portion 5 integrally formed with a cylindrical portion 4 of a power supply electrode portion 3. In this manner, the mechanical strength of the bottom portion 2 is increased. The thickness dW of the side wall portion 1 is measured such that a line normal to a tangent of an inner wall of the side wall portion 1 vertically cuts the side wall portion 1. The thickness dB of the bottom portion 2 is set such that a line normal to a tangent of the inner wall of the bottom portion 2 vertically cuts the bottom portion 2.
The thickness dB of the bottom portion 2 of the conventional heating resistor of this type is greater than the thickness dW of the side wall portion 1. In this sense, a resistance of the bottom portion 2 is much lower than that of the side wall portion 1. In addition, by forming slits 6 in the side wall and bottom portions, a zigzag-shaped heating conductor 7 is obtained. The width of the heating conducting 7 decreases toward the center of the bottom portion 2, since the bottom portion 2 is integrally formed with the cylindrical portion 4 which supports the bottom portion 2 and is connected to the power supply electrode portion 3.
Furthermore, since the thickness dB of the bottom portion 2 increases toward the center thereof, the cross-sectional area of the heating conductor 7 is substantially uniform from the periphery toward the center of the bottom portion 2. Unlike the case of the side wall portion 1, the bottom portion 2 does not have a resistance gradient. In other words, the bottom portion of the conventional heating resistor which has the above-mentioned bottom structure does not substantially contribute to heating of the crystal material in the melting pot coaxially surrounded by the heating resistor.
In the conventional resistor of the type described above, therefore, the temperature of only the crystal material portion in the vicinity of the side wall portion of the melting pot is raised, since the bottom portion 2 does not contribute to heating. As a result, a radial temperature gradient of the melt tends to be increased.
In an LEC monocrystalline pulling apparatus disclosed in Japanese Patent Publication No. 52-39787, a heater structure is shown wherein a temperature gradient along the direction of height of the heater is considered. However, even in this heater, as shown in FIGS. 5 and 6, the lower portion of the heater has a lower temperature than that of the upper portion thereof. As a result, the bottom portion of the heater substantially fails to sufficiently contribute to heating the molten material. In other words, this prior art improves only the temperature gradient along the direction of height of the melting pot. However, a concept for improving the radial temperature gradient of the melting pot is neither explicitly nor implicitly shown in Japanese Patent Publication No. 52-39787. Even in this heater, the radial temperature gradient of the crystal material tends to be increased.
The present inventors have made extensive studies and determined the fact that a large radial temperature gradient of the molten crystal material in the melting pot results in an increase in thermal strain in a crystal plane of a pulled crystal, dislocations in the crystal, nonuniform distribution of the dislocations, or nonuniform distribution of the impurity.
Especially in the LEC method, in order to decrease decomposition and scattering of elements with a high vapor pressure from the molten crystal material, the molten crystal material is sealed by a liquid sealing agent such as B.sub.2 O.sub.3, and the single crystal is pulled up while the molten crystal material is pressurized by an inert gas at a high pressure.
High heat radiation from the melting pot occurs due to convection of the high-pressure gas, so that the radial temperature gradient of the molten crystal material in the melting pot in the LEC method becomes even greater than that in the CZ method.
For this reason, when a GaP single crystal is manufactured by the LEC method using the conventional heating resistor, a dislocation density in a wafer plane of the resultant single crystal is as high as 1.times.10.sup.5 cm.sup.-2. In addition, the dislocation distribution in the wafer plane is regarded as being nonuniform.
FIG. 2 shows a microphotograph of a crystal structure when a single crystal is pulled up by the LEC method using the conventional heating resistor, the resultant single crystal rod is cut into wafers, and wafer is then polished and etched by an RC etching solution, thereby generating etching pits (circular portions) corresponding to dislocations of the crystal. The high dislocation density wwithin the wafer plane and the nonuniform distribution of the disclocations result in degradation of the light-emitting efficacy and in variations in the light-emitting characteristics of a light-emitting diode obtained using such a wafer.